GLACIAL CYCLES
Persistent influence of obliquity on ice age
terminations since the Middle Pleistocene transition
Petra Bajo1,2,3, Russell N. Drysdale1,4*, Jon D. Woodhead^5 , John C. Hellstrom^5 , David Hodell^6 ,
Patrizia Ferretti7,8, Antje H. L. Voelker9,10, Giovanni Zanchetta^11 , Teresa Rodrigues9,10, Eric Wolff^6 ,
Jonathan Tyler^12 , Silvia Frisia^13 , Christoph Spötl^14 , Anthony E. Fallick^15
Radiometric dating of glacial terminations over the past 640,000 years suggests pacing by Earth’s
climatic precession, with each glacial-interglacial period spanning four or five cycles of ~20,000 years.
However, the lack of firm age estimates for older Pleistocene terminations confounds attempts to
test the persistence of precession forcing. We combine an Italian speleothem record anchored by a
uranium-lead chronology with North Atlantic ocean data to show that the first two deglaciations of
the so-called 100,000-year world are separated by two obliquity cycles, with each termination starting at
the same high phase of obliquity, but at opposing phases of precession. An assessment of 11
radiometrically dated terminations spanning the past million years suggests that obliquity exerted a
persistent influence on not only their initiation but also their duration.
A
major challenge of testing the orbital
(Milankovitch) theory of the ice ages
is the uncertainty associated with the
chronology of marine records. Orbital
solutions are very accurate over the Pleis-
tocene ( 1 ), but the age profile of deep-ocean
sediments, where much of the evidence for
global ice volume changes is preserved, often
has large errors. Astronomical tuning of ocean
records renders any test of the Milankovitch
hypothesis invalid because of circular logic.
Testing theories of orbital forcing ultimately
requires ocean sediment records firmly an-
chored in absolute time.
A poorly understood feature of Pleistocene
glacial-interglacial (G-IG) cycles is the change
in the period of terminations—the relatively
rapid switches from glacial to interglacial
climate—during the Middle Pleistocene tran-
sition (MPT) 1.25 to 0.7 million years ago
(Ma) ( 2 – 7 ). Evidence from ocean sediments
shows that most terminations occurred every
~40,000 years (40 kyr) prior to the MPT but
averaged ~100 kyr in the post-MPT interval
( 5 ). Although the precise mechanisms for this
switch remain unclear ( 4 – 6 ), recent studies
highlight the critical interval of marine iso-
tope stages (MIS) 24–22, when major changes
in ocean circulation and ice sheet dynamics
occurred ( 7 , 8 ). This interval includes a“failed
termination”at the MIS 24–23 transition, the
residual ice from which probably contributed
to the steplike increase in global ice volume
observed over the subsequent MIS 22 glacial
(the“900-ka event”)( 5 , 9 ). Accordingly, the
interval bounded by the MIS 26–25 and 22– 21
transitions—terminations XII and X (TXII and
TX), respectively—is often erroneously consid-
ered to be the first“100-kyr cycle”( 7 ).
The transition to the“100-kyr world”oc-
curred without considerable shifts in astro-
nomical parameters ( 4 , 7 , 8 ), implying that
internal forcing changed the way the Earth
system responded to orbital variations. The
~40-kyr period for pre-MPT G-IG cycles ( 5 , 10 )
suggests pacing by changes in Earth’saxial
tilt, or obliquity ( 1 ), which affects the degree of
seasonality in a given year. At high obliquity,
the polar latitudes in both hemispheres re-
ceivemoresummerinsolation, potentially in-
ducing substantial ice sheet ablation ( 11 ). The
dominance of a ~100-kyr periodicity for post-
MPT terminations has been linked to forcing
by changes in Earth’seccentricity( 1 , 12 ), but
each ~100-kyr interval is more likely a cluster
of climatic precession (herein, precession)
and/or obliquity ( 8 , 13 , 14 ) cycles whose sum
averages to ~100 kyr when viewed over the
long term. This is supported by an Asian
monsoon speleothem record spanning all
terminations over the past 640 kyr ( 15 ), which
shows a spacing of four or five precession
cycles. Precisely what happened, in terms of
forcing, between the MPT and TVII [~635
thousand years ago (ka)] remains unclear, yet
the answer may assist in our understanding
of the MPT itself.
Studies focusing on G-IG cycles that traverse
the MPT ( 7 , 8 , 13 ) have relied on stacked records
of deep-ocean benthic oxygen isotope (d^18 O)changes ( 5 , 13 ), which are driven primarily
by variations in global ice volume ( 10 )but
which also record a prominent deep-ocean
temperature component ( 7 ). Given the inability
to directly date marine sediments beyond the
limits of radiocarbon dating, and given the
phase uncertainties between the benthic ice
volume–proxy record and astronomical (or
other) tuning targets, precisely datable archives
are required. We independently determined
the age of terminations across the MPT by
tying the radiometric chronology from a
speleothemd^18 O time series to North Atlantic
ocean sediment records. We then compared
our results with astronomical and insolation
parameters ( 1 , 9 , 13 , 16 ) for terminations since
640 ka ( 15 , 17 ).
Our speleothem record comes from Corchia
Cave (Alpi Apuane, Italy) ( 18 – 20 ) and spans
the interval ~970 to ~810 ka, encompassing
two complete terminations (TXII and TX) and
one uncompleted termination ( 7 , 8 ). A com-
posited^18 O time series derived from four
stalagmites (CC8, CC30, CC119, and CC122)
and a subaqueous speleothem (CD3) (Fig. 1)
wasanchoredinabsolutetimeusingtheU-Pb
method ( 18 , 20 – 22 ) (figs. S1 and S2 and table
S1). Almost the entire record is replicated, and
concordance between both the individual
stalagmite age models (fig. S3A) and the over-
lapping stable-isotope profiles (fig. S4A) allows
all U-Pb ages to be placed onto a common
depth scale to produce a composite age-depth
model ( 18 , 20 ) (fig. S3B). After accounting for
all sources of random and correlated uncer-
tainties ( 20 ), the average model-age precision
over the whole record is <7 kyr (95% con-
fidence interval) (fig. S3C).
The climate at Corchia Cave has strong tele-
connections with circulation changes in the
North Atlantic ( 19 , 23 ), from where well-
resolved marine records of glacial terminations
have emerged ( 24 , 25 ). Previous studies have
shown that Corchia speleothemd^18 Otracks
changes in sea surface temperature (SST) re-
corded off the Iberian margin ( 19 , 26 ) through
the effect of SST on moisture advection to, and
ultimately rainfall amount above, the cave site.
However, during terminations, the link be-
tween regional SST and speleothemd^18 Ois
overridden by large decreases in thed^18 Oof
surface ocean water (d^18 Osw)causedbycollapse
of continental ice sheets ( 18 ). This flux of low
d^18 Oswvalues introduces a“source effect”that
is captured in rainfalld^18 O at the cave, then
recorded in its speleothems ( 27 ). Similar toRESEARCH
Bajoet al.,Science 367 , 1235–1239 (2020) 13 March 2020 1of5
(^1) School of Geography, University of Melbourne, Carlton, Victoria 3053, Australia. (^2) Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales 2234, Australia.
(^3) Croatian Geological Survey, 10000 Zagreb, Croatia. (^4) Laboratoire EDYTEM–UMR5204, Université de Savoie Mont Blanc, 73376 Le Bourget du Lac, France. (^5) School of Earth Sciences, University
of Melbourne, Parkville, Victoria 3010, Australia.^6 Godwin Laboratory for Palaeoclimate Research, Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK.^7 Istituto per
la Dinamica dei Processi Ambientali, Consiglio Nazionale delle Ricerche (IDPA-CNR), Venice 30172, Italy.^8 Dipartimento di Scienze Ambientali, Informatica e Statistica, Università Ca’Foscari,
Venice 30172, Italy.^9 Instituto Português do Mar e da Atmosfera (IPMA), Divisão de Geologia e Georecursos Marinhos, 1495-165 Alges, Portugal.^10 Centre of Marine Sciences (CCMAR), University
of the Algarve, 8005-139 Faro, Portugal.^11 Department of Earth Sciences, University of Pisa, Pisa 56100, Italy.^12 Department of Earth Sciences, University of Adelaide, North Terrace, South
Australia 5005, Australia.^13 School of Environmental and Life Sciences, University of Newcastle, Callaghan, New South Wales 2308, Australia.^14 Institute of Geology, University of Innsbruck, 6020
Innsbruck, Austria.^15 Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, Scotland, UK.
*Corresponding author. Email: [email protected]